I think it depends on which way you view your aspirations.
If you want to develop your expertise in a certain industry, then understanding PLC functionality will probably help but not be the approach that will develop expertise in that industry. As the acronym indicates, a Programmable Logic Controller is based on logic, and this logic will fit into the application effectively when someone with a high level of expertise and knowledge of that industry can either programme the PLC to do what they want or they instruct someone with good PLC knowledge what they want.
On the other hand; if you want to develop your knowledge based on the PLC product and utilise across many types of industries, then becoming an expert in that product usually means working for companies who specialise in PLCs.
My basic point is, don't think that doing a PLC course will give you any more insight into a certain industry other than a logical view from the product. The difference a PLC will make in any type of industry is drawing from experience in that industry to ensure the logic functions work.
I have seen many excellent PLC programmers fail miserably because they do not understand the application and likewise, people who really know their industry who try and programme some wonderful products, miss some real innovative and useful functions because they simply do not understand the product.
It's a combination.
Coming back to your main point, gaining knowledge in anything is never wasted. Go for it but remember that knowing how a plc functions is not knowing how an application works, and vice-versa.
Just my 2 cents.